Liquid Crystal Displays (LCDs) are widely used to display information. LCDs are used for direct view displays, as well as for projection type displays. Electro-optical modes employed are e.g. the twisted nematic mode (TN), the super twisted nematic mode (STN), the optically compensated bend mode (OCB), the electrically controlled birefringence mode (ECB), or the vertically aligned mode (VA) with their various modifications, using an electrical field which is substantially perpendicular to the substrates, respectively to the liquid crystal (LC) layer. Furthermore, there are electro-optical modes employing an electrical field substantially parallel to the substrates, respectively to the LC layer, like for example the In-Plane Switching mode (IPS).
A new type of LCDs is disclosed in WO 02/93244 A1 and DE 10217273 A1 and comprises an LC cell with an electrically switchable LC medium that is operated in an optically isotropic phase, for example the blue phase or the isotropic phase, where it becomes birefringent when an electric field is applied due to the Kerr effect. Interdigitated electrodes on one side of the LC cell create an in-plane electric field parallel to the plane of the cell, which aligns the LC molecules in a planar texture along the electric field lines. These LCDs are hereinafter also referred to as “isotropic mode LCD”.
Another new type of LCDs is disclosed in WO 01/07962. It contains an LC switching cell comprising at least one polariser and an LC medium which has an initial alignment where the LC molecules are aligned essentially parallel to the cell substrates and are essentially untwisted, i.e. essentially parallel or antiparallel to one another. The LC molecules are realigned from their initial alignment by a corresponding electric field. In case of LC materials of negative dielectric anisotropy, the electric field is aligned essentially parallel to the substrates. In case of LC materials of positive dielectric anisotropy the electric field is aligned essentially perpendicular to the substrates. These LCDs are hereinafter also referred to as “new mode LCD”.
Further known display types are for example scattering displays, guest-host displays or cholesteric displays like SSCT (surface stabilized cholesteric texture).
LCDs can be operated as multiplexed or matrix displays. Typical examples of multiplexed displays are TN and STN displays. Typical examples of matrix displays are TN, IPS, OCB, ECB or VA displays. In matrix LCDs (MLCD), examples of nonlinear elements which can be used to individually switch the individual pixels are active elements like transistors. This is then referred to as an “active matrix”. A differentiation can be made between two types:    1. MOS (metal oxide semiconductor) transistors on silicon wafers as substrate,    2. Thin-film transistors (TFT) on a glass plate as substrate.
In the case of type 1, the electro-optical effect used is usually dynamic scattering or the guest-host effect. The use of monocrystalline silicon as substrate material restricts the display size, since even the modular assembly of various part-displays results in problems at the joins.
In the case of type 2, which is preferred, the electro-optical effect used is for example the TN or IPS effect. A distinction can be made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. Intensive research efforts are being made worldwide in the latter technology.
The TFT matrix is applied to the inside of one glass plate of the display, while the inside of the other glass plate carries for example the transparent counterelectrode. Compared with the size of the pixel electrode, the TFT is very small and has virtually no adverse effect on the image. This technology can also be extended to fully colour-compatible image displays, in which a mosaic of red, green and blue filters is arranged in such a manner that each filter element is located opposite a switchable pixel.
The term MLCD as used here covers any matrix display containing integrated nonlinear elements, i.e. in addition to the active matrix, also displays containing passive elements such as varistors or diodes (MIM=metal-insulator-metal).
MLCDs are particularly suitable for monitor or TV applications or for high-information displays e.g. in automobile or aircraft construction.
One drawback of LCDs is that they often show a limited viewing angle performance. For example, in specific directions the dark state of the display exhibits light leakage which results in contrast reduction. Also, the light state luminance is often reduced and colouration can occur. Furthermore, the contrast especially at large viewing angles is often limited which is a disadvantage especially in large area applications such as monitors and television.
Therefore, typically one or more optical retardation or compensation films are applied to the LCDs in order to compensate for light-leakage and to improve the LCD properties like the viewing angle characteristics, luminance, contrast and colour.
Typical retardation films of prior art comprise optically isotropic polymers like polyethyleneterephthalate (PET), polyvinylalcohol (PVA) or polycarbonate (PC), or slightly birefringent polymers like di- or triacetylcellulose (DAC, TAC), which become birefringent or wherein the birefringence is increased due to uniaxial stretching or compression.
Particularly suitable as compensators or retarders are anisotropic polymer films comprising a polymerised or crosslinked LC material with uniform orientation, like for example films with planar, homeotropic, tilted, splayed or helically twisted orientation. Such films and their use as compensators in LCDs are described for example in WO 98/04651 (planar films), WO 98/00475 (homeotropic films), WO 98/12584 (tilted and splayed films), GB-A-2 315 072 and WO 01/20394 (helically twisted films). Compensators comprising combinations of such films are disclosed for example in WO 01/20392, WO 01/20393, WO 01/20394 and WO 01/20395.
The retardation or compensation films are typically positioned in an LCD device between the polarisers and the display cell that comprises the switchable LC medium. They can also be laminated onto the polarisers. However, the use of such retardation films increases the total thickness of the device, and increases the manufacturing efforts and costs for the production of the display. Moreover, a display where the optical retardation film is attached outside the substrates forming the LC cell usually suffers from parallax problems, which can severely impair viewing angle properties.
One aim of the present invention is to provide optical retardation films for use in LCDs, and to provide LCDs, which do not have the drawbacks mentioned above. Other aims of the invention are immediately evident to those skilled in the art from the following description.
It was found that these aims can be achieved, and the above mentioned drawbacks can be avoided by placing an optical retardation film not outside the switchable LC cell of the LCD, but between the substrates forming the switchable LC cell and containing the switchable LC medium (“incell application”). It was also found that particularly suitable for such incell application is a film comprising polymerised LC material.